Over the recent years, there has been advanced development of a ferroelectric memory (FeRAM) which retains information on a ferroelectric capacitor by utilizing an inversion of polarization of a ferroelectric substance. The ferroelectric memory is a nonvolatile memory from which the retained information thereof does not disappear even when switching off a power source, and is focused particularly because of its being capable of realizing high integration, high-speed drive, high durability and low power consumption.
A material of a ferroelectric film in the ferroelectric capacitor includes, for example, a ferroelectric oxide having a Perovskite crystalline structure, such as a PZT (Pb (Zr, Ti)O3) film and a SBT (SrBi2Ta2O9) film on the order of 10-30 μC/cm2, which has a large quantity of residual polarization. It has hitherto been known that this type of ferroelectric film is deteriorated in terms of a characteristic of the ferroelectric substance due to a water content getting permeated from outside via an inter-layer insulating film exhibiting a high affinity with the water such as a silicon oxide film. Namely, the permeated water content or the water content remaining in the film is dissolved into hydrogen and oxygen in a high-temperature process when the inter-layer insulating film and a metal wiring film are grown. Then, if the hydrogen permeates the ferroelectric film, the hydrogen reacts on the oxygen in the ferroelectric film, and an oxygen defect is formed in the ferroelectric film, with the result that crystallinity declines. Further, the same phenomenon occurs also from long-term use of the ferroelectric memory. As a result, there occurs deterioration of performance of the ferroelectric capacitor, such as decreases in quantity of the residual polarization and in dielectric constant of the ferroelectric film. Further, the performance of a transistor etc might be deteriorated without being limited to the ferroelectric capacitor.
Aluminum oxide Al2O3 for preventing the permeation of the water content and the hydrogen has hitherto been used for coping with the deterioration such as this. For example, a conventional technology is that the aluminum oxide is formed so as to cover the ferroelectric capacitor, and protects the ferroelectric capacitor so that the water content and the hydrogen do not permeate an interior of the ferroelectric substance. For example, as in FIGS. 3 through 6, the following processes are carried out.
(1) After forming the ferroelectric capacitor, an aluminum oxide film (which is also referred to as an AlO film) is formed so as to embrace the ferroelectric capacitor, thereafter, the aluminum oxide film is covered with an inter-layer insulating film, and the surface is flattened (FIG. 3).(2) A resist is coated, and a resist pattern becoming a mask when forming a bulk contact hole is formed (FIG. 4).(3) With the resist pattern serving as the mask, the bulk contact hole is formed by etching (FIGS. 5 and 6).
FIGS. 1 and 2 illustrate a problem of the contact hole formed in these processes. As in FIGS. 1 and 2, a shape and a depth of the hole formed by etching become ununiform at the center of the wafer and in the periphery of the wafer as the case may be.
Further, the ferroelectric capacitor is formed generally between a transistor layer and a wiring layer, and hence the bulk contact hole connecting the transistor layer to the wiring layer has an extremely high aspect. Moreover, FIG. 1 exemplifies a planar type ferroelectric capacitor, however, this problem similarly occurs in a stack type ferroelectric capacitor.
[Patent document 1]
    International Publication WO98/01895[Patent document 2]    Japanese Patent Laid-Open Publication No. 2006-156932[Patent document 3]    Japanese Patent Laid-Open Publication No. 2003-224207